Couplings to connect to a flared end of a tube have often been a two-part fitting of a body and a nut. However, this requires flaring the end of the tubing, and in many cases this is inconvenient or difficult. Accordingly, straight tubing or non-flared tubing has also been attempted to be secured to a coupling, and two-part couplings have been utilized, as in U.S. Pat. No. 927,388, and on small diameter tubes, as in U.S. Pat. No. 3,375,026. Three-part tube couplings for non-flared tubes have been more common, including those disclosed in U.S. Pat. Nos. 3,025,086; 3,454,290; 3,834,743; and 4,162,802. Also, The Weatherhead Company has disclosed a three-part flareless tube fitting in its 2600 Series of tube fittings. Four-part tube fittings have also been disclosed, as in U.S. Pat. Nos. 3,493,250 and 3,685,860.
These tube fittings have met with differing degrees of success, usually for low and medium pressure, because at high pressure it was difficult to make certain that the tube did not blow out of the tube coupling.
In air conditioning, refrigeration and heat pump units which are stationary units, there is still the problem of vibration and a desire to seal the refrigerant without any leakage. In such a case, the typical connection for the refrigerant tubing was to use some form of hermetic seal rather than a threaded coupling seal. Such hermetic seals included inert gas brazing or silver solder rather than relying on a threaded coupling which might fail after a long period of use and require a service call, which was usually quite expensive for the manufacturer or installer. The rather slight vibration caused by the running of the compressor could cause metal fatigue and failure of threaded couplings; hence, these were generally avoided.
Automotive air conditioning added additional problems to try to obtain a leak-free tubing connection, namely: the wide temperature range from below 0.degree. F. to above 100.degree. F. under the hood of a modern automobile; the vibration of the automobile engine; and the road shocks as the automobile hit chuck holes, etc.
Cryogenic air conditioning apparatus utilized in industry and medical applications created possibly even worse ambient conditions, with temperatures varying widely from ordinary room temperature of 70.degree. F. down to -80.degree. F., or even -120.degree. F., together with the vibration from the compressor. As a result, in such automotive and cryogenic air conditioning applications, the practice almost universally has been to utilize some form of hermetic seal on the tubing connections rather than to trust to any form of a threaded connection. Nevertheless, failures have occurred out in the field, where it was up to the service man make a repair joint without access to any production equipment as found in the usual manufacturing facility, e.g., without benefit of any inert gas brazing equipment.
A difficulty with the two-part hose couplings disclosed in the prior art U.S. Pat. No. 927,388 was that the external sleeve thereon was slotted longitudinally to permit the sleeve to be compressed radially inwardly, and thus this effectively cut in half the length of the leakage path, which could be satisfactory for a low pressure hose but unsatisfactory for a high pressure tube connection. The two-part tube coupling of the prior art U.S. Pat. No. 3,375,026 was only for use with very small diameter tubing, e.g., 1/16 inch outside diameter, and the force required to longitudinally move the compression sleeve would be extremely high in larger sizes of tubing. Also, such coupling could be one where vibration would vibrate loose such compression sleeve.
The difficulty with the three-piece tube couplings employing a separate external sleeve was that upon turning the nut, the friction between the external sleeve and the nut tended to rotate the sleeve, which tended to twist the tube, and if the tube were of soft material, the tube could be twisted off before a tight seal was made. Further, with aluminum tubing, a small score mark, as an annular score around the tube, weakened the tube sufficiently so that subsequent vibration could cause metal fatigue and breakage of the tube at such annular score line.
The four-part tube couplings of the prior art were intended for use with plastic tubing, which had little compressive strength in a radially inward direction and required the insertion of an inner sleeve, yet this created an additional junction whereat leakage could occur rather than minimizing the locations where leakage could occur.